.box{
  position: fixed;
  width: 100%;
  height: 100%;
  top:0;
  left:0;
  background: rgba(0,0,0,0.5);
  
}
.content{
  width: 600px;
  height: 400px;
  background: #fff;
  margin: 50px auto;
  text-align: center;
}